a) Field of the Invention
The invention is directed to a method and an arrangement for plasma-based generation of soft x-radiation, particularly for the generation of extreme ultraviolet (EUV) radiation, in which a target flow of defined portions which is made available in a reproducible manner is interacted with a pulsed energy beam for exciting radiation-emitting plasma, wherein the interaction results in the generation of a radiation-emitting plasma. The invention is preferably applied in radiation sources with high repetition rates, preferably in radiation sources for semiconductor lithography.
b) Description of the Related Art
Plasma-based radiation sources in which the plasma is generated by introducing energy into a target preferably comprise a target flow that is injected into a vacuum chamber. The plasma is then generated at a short distance from the place of injection (nozzle) by interaction with a pulsed energy beam. Control of the process parameter of temperature is critically important particularly when using a target flow of liquid xenon at temperatures around −100° C. in order to ensure the stability of the target flow. However, the stability of the target is drastically reduced by the heating and erosion of the target nozzle over increasing operating periods or when the pulse rate of the plasma excitation is increased, so that the nozzle only has a short life.
In the prior art relating to the generation of radiation by plasma generation by means of an energy beam (usually a laser beam), plasma generation from mass-limited targets has found acceptance because such targets minimize unwanted particle emission (debris) compared with other types of targets. A mass-limited target is wherein the particle number in the region of interaction between the target and energy beam is limited to the order of magnitude of the ions used for generating radiation. A droplet generator is often used to generate mass-limited targets.
In this connection, EP 0 186 491 B1 describes the excitation of individual droplets, i.e., exactly one droplet is impinged upon per energy pulse. The droplets have the same order of magnitude as the laser focus. Because of constantly occurring variations in the droplet frequency, it is necessary to detect the droplet target and to synchronize with the laser pulses.
Further, targets in the form of clusters (U.S. Pat. No. 5,577,092), gas puffs (H. Fiedorowicz, SPIE Proceedings, Vol. 4688, 619) or aerosols (WO 01/30122) have been described for plasma generation. However, the average density of such targets in the focus volume is substantially less than in liquid targets or solid targets because the target comprises microscopic particles or is in gaseous form. Further, the target divergence is generally so big (opening angle of several degrees) that the average target density decreases rapidly with increasing distance from the nozzle and an efficient coupling in of the energy beam is possible exclusively in the immediate vicinity of the nozzle. The disadvantageous stressing of the nozzle mentioned above is accordingly inevitable.
While devices with a continuous target jet (liquid or frozen jet) such as those described in WO97/40650, for example, allow a relatively large working distance from the nozzle, they are susceptible to shock waves. This means that the coupled-in radiation-generating energy pulse causes hydrodynamic disturbance extending relatively far along the jet axis and the characteristics of the continuing jet for optimal plasma generation and radiation generation are impaired. This disturbance prevents a high pulse repetition frequency because it is necessary to wait for the disturbance to die away for the next pulse.